1. Technical Field
The present invention relates to a functional device and a manufacturing method thereof, and particularly, to a functional device in which a functional structure of an MEMS (microelectromechanical system) or the like is disposed in a cavity configured in a substrate, and a manufacturing method thereof.
2. Related Art
In general, an MEMS (microelectromechanical system) acting as a functional device is manufactured as various electronic parts utilizing a micropatterning technology used in a semiconductor manufacturing process or the like. In many cases, at least one portion of MEMS structures is contained and disposed inside a cavity configured in a substrate, and the cavity is hermetically sealed from above by a cover when necessary, and put into a condition in which it is depressurized and sealed, or a condition in which a specialty gas is sealed in it.
For example, in JP-A-2004-314292, a sealing structure is described which is configured by a method of, after forming an MEMS structure together with a sacrifice layer, and forming an insulating layer on the MEMS structure, forming a first sealing layer, which includes a vent, on the insulating layer, etching away the insulating layer and the sacrifice layer through the vent, forming a cavity and, after introducing a gas into the cavity, by forming a second sealing layer on the first sealing layer, closing the vent.
Also, in JP-A-2006-263902, a method similar to the heretofore described one is described whereby, in a structure in which a semiconductor integrated circuit device (CMOS) and an MEMS structure are monolithically configured, a cavity is formed at the same time, using a MOSFET wiring formation technology.
However, in the structure in which the cavity is hermetically sealed with the MEMS structure disposed therein in the way heretofore described, for example, in a case in which a first cover layer including apertures is formed, and a cavity is formed by an etching through the apertures, when the cavity is washed with water after the etching, and subsequently dried, the first cover layer may become depressed inward along with a lowering of a water level in the cavity. Also, in a case in which a second cover layer is subsequently laminated onto the first cover layer, and the cavity is depressurized and sealed, a cover may be depressed inward by an external pressure due to a pressure difference between inside and outside. This kind of depression of the cover, in some cases, causing a poor operation due to the cover making contact with the MEMS structure, or the like, may result in a reduction in an electronic device manufacturing yield, or a reduction in a product quality.
Furthermore, as an aspect of the cover covering the cavity also varies depending on a difference in thermal expansion caused by a layer structure of the cover, a change in manufacturing conditions, or the like, a deformation, such as the inward depression or an outward protrusion, may be caused not only at the heretofore described drying time, and depressurization and sealing time, but by another factor such as an internal stress of a laminate structure, and this may affect the product quality, such as by causing a variation in device characteristics.